Alternatively, to determine if SOD1 acts upstream we tested for rescue of the mutant SOD1-induced motor phenotype by overexpressing WT TARDBP or FUS. Overexpression of mutant SOD1 mRNA in zebrafish embryos has been shown to cause shortening and premature branching of axonal projections from the motor neurons in the spinal cord [8] and is consistent with the toxic gain of function observed in ALS [2], [7], [8]. Here, SOD1 is linked to amyotrophic lateral sclerosis.